Field of Invention
This invention relates to developing units for developing an electrostatic latent image. Particular embodiments relate to a developing unit for developing an electrostatic latent image with dry two-component developer material.
Description of Related Art
In electrophotographic printing machines an electrostatic charge image is formed on a dielectric recording member that may be a photoconductive dielectric recording member which, after being uniformly charged, is image-wise exposed to conductivity-increasing radiation, producing thereby a “direct” or “reversal” toner-developable charge pattern on the recording member.
Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier particles having toner particles adhering thereto. A single component developer material typically comprises toner particles having an electrostatic charge so that they will be attracted to, and adhere to, the latent image on the photoconductive surface. The present invention is particularly useful for two component systems and hybrid systems using two component developer materials.
When a two-component developer material is used, toner particles are mixed with larger magnetizable carrier particles. The toner particles adhere to the magnetizable carrier particles by electrostatic attraction force. The electrostatic charge of the toner and carrier particles is obtained triboelectrically by agitation.
A developing unit applies the toner-carrier mixture to the surface carrying the electrostatic charge image, wherein toner and magnetizable carrier particles are mixed and a layer of the toner-carrier mixture, also referred to as developer material, is picked up by a developer member such as a rotating sleeve or drum having magnets inside, forming a so-called magnetic brush on magnetic roller. On rotating the magnetic roller, the toner particles still adhering to the magnetically attracted carrier particles are brought into a developing zone wherein the toner particles are separated from the carrier particles by the electrostatic attraction forces of the electrostatic latent image to be developed and transfer to the latent electrostatic charge image. A developing bias voltage of suitable polarity applied between the magnetic brush and the recording member to be developed may determine whether the development is a “direct” or “reversed” development.
The magnetic brush has to be supplied with fresh toner-carrier mixture. This is normally done by an agitator, e.g. a paddle wheel, projecting or scooping up toner-carrier mixture onto the magnetic brush from a housing holding the developer material. The partly exhausted developer is returned to the bulk of developer material contained in the housing and has to be thoroughly mixed timely with freshly added toner particles to keep the toner-carrier weight ratio within acceptable limits for obtaining consistent development results.